The present invention relates to an optical structural film of backlight in a liquid crystal display device (LCD), in particular, to an enhanced prism film.
Optical structural films of the backlight in a LCD are typically used for enhancing the brightness and broadening the viewing angle of the LCD. FIG. 11 is a schematic view showing a conventional optical structural film of a backlight with an edge illumination structure. As shown in FIG. 11, the optical structural film of the backlight comprises a light guide plate 10, a diffusion film 20 and a prism film 30. A light source 40 disposed on at least one side of the optical structural film emits a light beam 50, which is introduced into the prism film 30 by the light guide plate 10. The prism film 30 then converges incident lights into a smaller range of angle for improving the brightness due to light refraction and total reflection of the prism film 30.
The conventional prism film is composed of a plurality of prism bars extending in a same direction, and each prism bar has a cross section shape of isosceles triangle. The conventional prism film can direct illumination lights only within a certain angle range to emit out of the prism film in the normal direction of upper surface of the prism film due to the straight line profile along axial direction of the prism bar, since outgoing lights incident on the upper surface of the prism bar with an angle exceeding the total reflection angle would not emit from the prism film due to the total reflection. Consequently, the brightness along the axial direction of the conventional prism film drops significantly as the viewing angle increases.